diff --git a/server/books/models/contentblock.py b/server/books/models/contentblock.py
index 1698ca23..f57ce2cc 100644
--- a/server/books/models/contentblock.py
+++ b/server/books/models/contentblock.py
@@ -44,6 +44,8 @@ def duplicate_entities_generator(new_module):
         # logger.debug(block)
         if content_type == "assignment":
             assignment = value.get("assignment_id")
+            if assignment is None:
+                return None
             # copy the assignment
             assignment.pk = None
             assignment.title = f"{assignment.title} (Kopie)"
@@ -56,6 +58,8 @@ def duplicate_entities_generator(new_module):
         if content_type == "survey":
             # logger.debug(value)
             survey = value.get("survey_id")
+            if survey is None:
+                return None
             # copy the survey
             survey.pk = None
             survey.title = f"{survey.title} (Kopie)"
@@ -165,13 +169,13 @@ class ContentBlock(StrictHierarchyPage, GraphqlNodeMixin):
         iterator = map(duplicate_entities, self.contents)
         # logger.debug("here is the iterator")
         new_contents = list(iterator)
+        cleaned_contents = [item for item in new_contents if item is not None]
         # logger.debug(new_contents)
         # we can't just insert a list here, we need a StreamValue data type
         # so we need to clear the list, then add each element in turn
         self.contents.clear()
         # like this, the internal methods of the SteamValue data type can work on the single elements
-        for content in new_contents:
-            # logger.debug(content)
+        for content in cleaned_contents:
             self.contents.append(content)
 
         # as an illustration